A fuse is a current interrupting device which protects a circuit by means of a current-responsive fusible element. When an overcurrent or short-circuit current of a predetermined magnitude and duration is conducted through the use, the fusible element melts, thereby opening the circuit. After having interrupted an overcurrent, the fuse must be located and replaced in order to restore service.
Fuses are typically employed in the electrical utility industry to protect distribution transformers, cables, capacitor banks and other equipment from damaging overcurrents. The fuses are arranged to disconnect the faulted equipment or circuit promptly from its source of supply before damage can occur. At the present time, two basic types of fuses are employed, the expulsion fuse and the current limiting fuse.
The earliest of these two types of fuses was the expulsion fuse. An expulsion fuse typically employs a relatively short length of a fusible element (within what is commonly termed a "fuselink") contained within a tubular enclosure that is part of a larger assembly known as a "fuseholder". The enclosure used in the expulsion type fuse is lined with an organic material, such as bone fiber. Interruption of an overcurrent takes place within the fuse by the deionizing and explosive action of the gases which are liberated when the liner is exposed to the heat of the arc that is created when the fusible element melts in response to the overcurrent. The operation of the expulsion-type fuse is characterized by loud noise and violent emission of gases, flame and burning debris, all of which pose a danger to personnel who may be in close proximity to the fuse when it operates. Because of its violent mode of operation, this type of fuse has generally been restricted to outdoor usage only. Even when employed outdoors, the expulsion fuse must be mounted well away from the equipment it is intended to protect, as well as other equipment, due to the explosive nature of its operation and its tendency to inject ionized gases into insulating spaces. Further, expulsion fuses mounted on distribution system poles have been known to initiate grass fires resulting from the flaming debris which may be expelled.
Another inherent disadvantage of the expulsion type fuse is that it requires 1/2 or sometimes 1 full cycle of current before the fuse clears a high current fault. During this time, the equipment the fuse is designed to protect must endure the full available fault current that is allowed to pass through the fuse to the equipment. Potentially damaging energy that will be dissipated in the equipment will be proportional to the formula I.sup.2 T, where I is the magnitude of the overcurrent and T is the time that the current condition exists. Additionally, the high current that an expulsion fuse allows to flow prior to its interruption at a system current zero tends to cause bothersome voltage dips upon the network, causing lights to flicker and sensitive computers and electronic equipment to suffer. Further, expulsion fuses may not clear the overcurrent condition soon enough to prevent sectionalizing fuses, reclosers or other protective relays and circuit devices from also sensing the overcurrent and responding by temporarily and sometimes permanently disconnecting other portions of the network. Additionally, the increased demand for electrical service has led to lower impedance distribution networks and the need for greater interrupting capabilities, capabilities which sometimes exceed those available through the use of expulsion fuses.
The limited interrupting capacity of expulsion-type fuses, coupled with their potentially dangerous mode of operation, their unsuitability for use within buildings or enclosures, their relatively slow clearing time, as well as other factors, prompted the development of the current limiting fuses. The current-limiting fuse has at least three features that have made it extremely desirable for use by the utilities:
(1) Interruption of overcurrents is accomplished quickly without the expulsion of are products or gases or the development of forces external to the fuse body because all the arc energy of operation is absorbed by the sand filler of the fuse and is subsequently released as heat at relatively low temperatures. This enables the current-limiting fuse to be used indoors, or even in small enclosures. Furthermore, since there is no discharge of hot gases or flame, only normal electrical clearances from other apparatus need to be provided.
(2) A current-limiting action or reduction of current through the fuse to a value less than that otherwise available from the power-distribution network at the fuse location occurs if the overcurrent greatly exceeds the continuous-current rating of the fuse. Such a current reduction reduces the stresses and possible damage to the circuit up to the fault or to the faulted equipment itself, and also reduces the shock to the distribution network.
(3) Very high interrupting ratings are achieved by virtue of its current-limiting action so that current-limiting fuses can be applied on medium or high-voltage distribution circuits of very high available short-circuit currents.
A current-limiting fuse typically consists of one or more fusible elements of silver wire or ribbon which are electrically connected at their ends to a pair of electrical terminations. The fusible elements require a minimum element length for proper fault current interrupting performance, and also require sufficient element cross sectional area in order to properly carry the normal or steady-state system currents. The assembly--consisting of the fusible element and end terminations--is placed in a tubular housing that is made of a highly temperature-resistant material, and the housing is then typically filled with high-purity silica sand and sealed. Terminals on the ends of the housing interconnect the fuse with the distribution network. The entire assembly is generally known as a current-limiting fuse.
When operating to clear a high magnitude fault current, the fusible element of a current limiting fuse melts almost instantaneously over its full length. If segments having reduced cross sectional areas are formed in the element, the element melts initially at these reduced area segments, followed by melting of the remaining length of the element. The resulting are rapidly loses heat energy to the surrounding sand. This energy melts or fuses the sand surrounding the element into a glass-like tunnel structure called a "fulgurite." The rapid loss of heat energy and the confinement of the arc by the molten glass fulgurite literally chokes off the current to a relatively small value. The current is quickly reduced to low levels, brought into phase with the system voltage and interrupted at the earliest-occurring current zero of the in-phase current.
Using a metallic ribbon as the high magnitude fusible element is quite common in higher current rated fuses. The ribbon form has the advantage over wire elements by having a larger surface area for thermal conductivity and radiation to the adjacent filler material. Consequently, for a given volume of conductor material, a ribbon element can have a higher steady-state ampere rating than a wire element, as well as improved interrupting characteristics. Ribbon also has the distinct advantage of lending itself to modification with perforations or notches in order to reduce its cross sectional area in order to provide the desired melt characteristics and exact arc-voltage generation control. When a current-limiting fuse using ribbon-type elements encounters a high-fault current, the ribbon portions having reduced cross-sectional area are heated rapidly to the melting point of the ribbon. This produces a fixed number of arclets in series and, thus, limits the magnitude of the arc-voltage spike produced at that instant. The ensuing arc formation continues to vaporize the remaining portions of the ribbon element and finally produces an arc which occupies the full length of the element path.
On low magnitude currents, such as those that might occur from high-impedance faults or sustained overloads, an entirely different phenomenon occurs. In these instances, the fusible element is heated slowly, and ultimately melts in a limited number or perhaps only one place. One or more short arcs begin and attempt to burn back longer sections of the fusible element. The very high heat of the arc again forms a fulgurite. However, because the initial arc length is short, and because the rate that the fusible element is burned back may not be fast enough to force a current interruption before the highly concentrated heat source destroys the effectiveness of the developed fulgurite, the fuse will fail to interrupt low magnitude currents. Consequently, to achieve interrupting capabilities for low magnitude fault currents, many of today's current limiting fuses employ a second fusible element in series with the primary element, where the second element is designed to fuse open in response to such low magnitude fault currents and to subsequently interrupt these currents.
Today, an important consideration to utilities in fuse selection and use relates to the ability of the fuse to be physically integrated within the utilities' existing network, and the ease and cost of installation and service. In present-day networks, expulsion fuseholders are typically installed in mountings which are known as "cutouts." Generally speaking, a cutout consists of a mounting having an insulating support designed to be mounted on a utility pole or crossarm and having a pair of spaced-apart terminals which are designed to receive and electrically engage a fuseholder, a switch assembly, or a combination thereof. When installed, the fuseholder or switch bridges the "gate" between the terminals of the cutout mounting.
The term "fuse cutout" usually refers to the combination of a cutout mounting, as described above, with a fuseholder. The fuseholder that is most typically employed in a fuse cutout is designed to be easily disengaged from the terminals of the cutout. One such fuseholder is the "dropout" type which is designed such that, upon actuation of the fuse, one end of the fuseholder becomes disengaged from the cutout mounting. When this occurs, the unrestrained end of the fuseholder rotates down and away from its normal bridging position between the mounting gate while the fuseholder remains supported from the mounting by its still-engaged end.
Expulsion type fuse cutouts offer a relatively convenient and low cost means of fusing, and thereby protecting electrical distribution systems. Further, the industry is adopting a dimensional standard for expulsion fuseholders and mountings, such that a fuseholder from one manufacturer will properly fit into the mounting of another manufacture. Further, these "interchangeable" cutouts are widely distributed throughout electrical distribution systems in this country, and large numbers of these cutouts are presently in service.
With increasing demands for electrical energy, more reliable service, higher levels of safety, the need for improved overvoltage protection of transformers and the desire for more compact systems, the continued use of expulsion fuse cutouts does not necessarily meet the needs of today's utilities. Many of the aforementioned problems associated with expulsion fuses could be overcome through the use of current-limiting fuses and fuseholder. However, the prior art current limiting fusing equipment has suffered from its own set of drawbacks.
Prior attempts to overcome some of the aforementioned problems are evidenced, for example, by the devices disclosed in U.S. Pat. Nos. 3,827,010 and 4,011,537. These devices provide a combination dropout assembly which include a current limiting fuse disposed in line and coupled in series with an expulsion-type fuse, such that a full range of protection is provided by the fuse cutout. However, the overall length of these devices is longer than the gate (the spacing between the terminals) of commonly used cutout mountings found in existing distribution systems. Thus, in order to effectively utilize these inventions, utilities would have to replace literally millions of cutouts presently in service. Such an approach would be prohibitive, not only from the standpoint of equipment cost, but also, and perhaps more significantly, in view of the monumental labor costs associated with the replacement of these cutouts. Further these devices do not adequately address or remedy fire hazards, spacing requirements, problems that may result from partial element damage to the series-connected device or the miscoordination which could occur when refusing one of the two series-connected sections.
Another prior art approach is illustrated in U.S. Pat. No. 3,863,187 which discloses an expulsion-type fuse in series with a current limiting fuse, but disposes the latter "outgate" such that it does not form a part of the dropout assembly. One shortcoming of this device is that the current limiting fuse is bolted in place, making replacement of the current limiting fuse difficult, particularly in adverse weather conditions. Compounding the difficulty is the fact that the network will typically be energized while maintenance personnel replace the fuse. Further, there is no method of readily determining whether the current limiting fuse has operated even where the expulsion fuse has operated and dropped open. Consequently, whenever the expulsion fuse portion of the device actuates, recommended practice is to replace both the expulsion fuse and the current limiting fuse. In addition, space, in excess of the normal expulsion fuse requirements, must be allocated for placement of the current limiting fuse. Also, proper electrical coordination of the two fuse sections must be maintained in order to ensure indication of a fuse operation and removal of voltage stress across the blown fuse by the dropout action of the expulsion fuse.
Still another fuse cutout is disclosed in U.S. Pat. No. 4,184,138 which discloses a design which contemplates offsetting the axes of a series-connected current limiting and expulsion fuse so that the combination will physically fit within existing interchangeable cutouts. However, like the three patents identified above, the invention suffers from many of the disadvantages inherent with the use of expulsion fuses, i.e., noise, expulsion of flaming arc products, coordination requirements, and the like. Additionally, the extra mounting hardware and mounting components for installing the apparatus is cumbersome and therefor undesirable. Further, as explained with respect to the U.S. Pat. No. 3,863,187, upon operation of the expulsion fuse, the current limiting fuse must also be tested, or replaced and later tested, thus eclipsing any significant cost savings.
Another prior art approach, one which does not rely on an expulsion fuse section, is shown in U.S. Pat. No. 3,611,240. This patent discloses a current limiting dropout fuseholder; however, the fuse is not designed to fit within the industry-standard interchangeable cutout mountings so prevalent in the industry today. Further, the dropout mechanism relies upon the use of an explosive charge which, upon detonation, releases the fuse for drop-open movement. Similarly, U.S. Pat. No. 3,825,871 also employs an explosive charge to initiate dropout of the fuse. Although such explosive charges have generally been successfully employed, it is not uncommon for such a fuse to fail to drop open after clearing a fault due to failure of the charge to detonate. Such failure is frequently due to the powder absorbing too much moisture to ignite after a prolonged period of service. No matter the reason for such failures, the failure of a dropout fuse to drop open after operation is a source of great frustration and delay as utility personnel are unable to locate the actuated fuses by simple visual observation, and must instead resort to more time consuming and less convenient means for detecting which fuses have operated. Further, the fuseholder that has failed to drop open remains subject to the voltage stress imposed by the energized network, making it susceptible to tracking and possible flash over.
Accordingly, despite the many advances made in fuse technology over the last decade, further advances would be welcomed by the industry. Specifically, due to the increased demand for use of current limiting fuses and the cost-driven necessity of employing existing cutout mountings, there exists a need for a full-range current limiting fuseholder sized so as to fit within the gate of interchangeable cutout mountings presently in-service. The current limiting fuseholder would be entirely of the nonexpulsion type to avoid potential danger to personnel, to eliminate the threat of starting a fire, and to allow the apparatus to be safely mounted closer to the protected equipment or to other structures, and would operate without the noise and voltage dip which accompanies expulsion fuse operation. Preferably, such a fuseholder would be of the dropout variety to provide indication of a fuse operation, to relieve voltage stress across a blown fuse and to allow ease of installation and maintenance. Ideally, the dropout mechanism would not be dependent upon an explosive charge for initiating the drop open movement of the fuseholder, but would be mechanically actuated and would consistently cause drop out in both low and high current-rated fuses on either low or high magnitude faults.